Researcher Q&A FAQ-Milky Way and other Galaxies

These questions have been answered by the scientists who are part of the Ask a High-Energy Astronmer program.

The Milky Way Galaxy (Our Own)

QUESTION:
How large is the Milky Way?

The disk of the Milky Way galaxy is about 100,000 light years in diameter (one light year is about 9.5 x 1015 meters), but only about 1000 light years thick.

Our Galaxy contains about 200 billion stars. Most of the stars are located in the disk of our galaxy, which is the site of most of the star formation because it contains lots of gas and dust.

The halo, which is a spherical cloud surrounding the disk, contains only about 2% as many stars as the disk. It contains old and cool stars, since it has little gas and dust.

Eric Christian and Samar Safi-Harb

QUESTION:
I studied a bit of astrophysics at UCLA. I was trying to recall the estimated rate of speed that our Sun is moving in relation to the center of the milky Way galaxy.

And is the whole Milky Way galaxy moving also? I would assume so, but was wondering if there is some calculations.

The Sun orbits the center of the Milky Way at about 250 km/second and it takes about 220 million years to complete an orbit.

The Milky Way is part of a group of galaxies known as the Local Group. All of these are moving relative to each other due to their gravitational interaction with speeds of around 100 km/s or less. Calculating the velocities of the galaxies in the Local Group is difficult because there are probably members that have not yet been discovered because they are too dim or are obscured by the plane of the Milky Way. The radial velocities relative to the Milky Way are found by measuring Doppler shifts in the spectra of stars in the galaxies. You will find more information at http://seds.lpl.arizona.edu/messier/more/mw.html

The Local Group is also moving at about 600 km/second relative to the cosmic microwave background. There's a nice picture of this at http://antwrp.gsfc.nasa.gov/apod/ap960205.html

Damian Audley and David Palmer
for the Ask a High-Energy Astronomer Team

QUESTION:
How many stars (or what percentage) in our galaxy are 4.5 billion years old (age of the Sun) or older?

The Sun is about 4.5 billion years old now, but its total life is estimated to be about 10 billion years. Most stars in the Milky Way live at least this long. And experts estimate that stars have been forming at a similar rate for most of the history of Milky Way (~10 billion years). This means about half the stars are older than 4.5 billion years old.

Koji Mukai

QUESTION:
What proof do astronomers have when saying our galaxy looks a certain way? All evidence would be indirect I would think. What is this evidence?

You have a very good point here: How can we have any idea what our galaxy looks like from the standpoint of an outside observer, when we ourselves are embedded in the middle of it? As you surmised, some detective work is required. If we were to rely only on optical light in trying to formulate an educated guess as to the answer, it would be next to impossible because of all the obscuration due to dust in the plane of our galaxy. When one goes to other wavelengths, however, the project becomes do-able.

The first work which mapped out the spiral structure of our galaxy was done at radio wavelengths by studying the "21 cm" line which is due to the "spin-flip" transition of the hydrogen atom. Basically, this means that the hydrogen atom can have slightly different energy states depending on whether the spin of the nucleus is parallel or anti-parallel with the spin of the whole atom. This change from one state to the other produces this emission, which passes through the dust. By investigating the strength of this emission as a function of point on the sky, early workers (i) determined that we live in a spiral galaxy, and (ii) mapped out the spiral arms. In addition to measuring the strength of the signal at different points of the sky, one also makes use of the Doppler shift of the signal to infer the velocity structure.

More recently it has become feasible to perform this exercise at other wavelengths. For instance, the COMPTEL instrument on the Compton Gamma Ray Observatory has mapped out the spatial distribution of radioactive aluminum 26 which produces an emission line at 1.8 million electron volts. To see what kind of galactic structure they have inferred, take a look at the figure in the following paper. (The paper itself is rather technical, but the figure just shows a picture of the spiral structure of the galaxy in our vicinity.) The paper is just 2 pages, with one figure.

(i) Go to this site http://adsabs.harvard.edu/abstract_service.html

(ii) In the author field enter: Chen, Gehrels, Diehl, Hartmann, then select the Boolean "and" search option, and click "send query". This will get you the following paper:

"On the spiral arm interpretation of COMPTEL 26 Al map features" Astronomy & Astrophysics Supplement Series vol. 120, 315. (1996)

It has just been pointed out to me that it may be difficult for you to access this reference in the manner I suggest if you are on other than a UNIX machine. In that event, it may be necessary to find a university library to get this paper.

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Of possible interest, here is the Multiwavelength Milky Way poster site http://adc.gsfc.nasa.gov/mw/milkyway.html
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J.K. Cannizzo, K. Smale, D. Palmer

QUESTION:
I am in 5th grade. Do you have any pictures of the Milky Way from above it?

There are no pictures of the Milky Way galaxy taken from above. That would require us or a space probe to be able to get far enough away. That hasn't yet been done. The furthest a space probe has gotten from the earth is Pioneer 10, which is now more than 10 billion km from the earth. (For comparison, Pluto is about 6 billion km away). However, Pioneer 10 would have to go 3 billion times farther to get far enough away to take a picture of the entire galaxy.

However, astronomers think they know what the Milky Way galaxy would look like from outside it. They have long thought that it is a spiral galaxy, like the Andromeda Galaxy or the Whirlpool Galaxy. However, recent research indicates it may be a barred spiral, like M91.

You can find a picture and a description of each of these galaxies on these web sites:

Andromeda: http://www.maa.mhn.de/Messier/E/m031.html
Whirlpool: http://www.maa.mhn.de/Messier/E/m051.html M91:
http://www.maa.mhn.de/Messier/E/m091.html

Jim Lochner (with help from Gail Rohrbach and Koji Mukai)

QUESTION:
I am curious to know why with the HST we can see the galaxies in the deep space image that are billions of light years away, but we can't study or even see the center of our own galaxy?

It's not true that we can't study or even see the center of our galaxy. This is a very active research topic for astronomers, particularly using radio, infrared, X-ray and gamma-ray telescopes.

As for seeing the Galactic center: a search in the "Astronomy Picture of the Day" site has turned up 3 nice pictures:

http://antwrp.gsfc.nasa.gov/apod/ap960605.html
http://antwrp.gsfc.nasa.gov/apod/ap970121.html
http://antwrp.gsfc.nasa.gov/apod/ap971111.html

However, it is true that it's hard to see the Galactic center with visible light. This is because of the dust in our own Galaxy, which can be seen in the first of the 3 "APOD" pictures above as dark patches in the Milky Way. Such dust clouds make stars behind them appear much fainter. Hope this helps. Best Wishes,
Koji Mukai

QUESTION:
I am studying about 21cm wavelength or 1420 MHz radio waves coming from the sky. Do you know where I can find information about sky mapping with a radio telescope at 21cm?

There is a map of the Milky Way in the 21 cm H line, and other wavelength bands at -

There are also links to key papers describing 21 cm studies. These links lead into the abstract service of the Astronomical Data System, from which you can search for similar papers.

http://dopey.haystack.edu/srt_html/srt.html

- which describes the Small Radio Telescope Project at the Haystack Observatory. They have been able to make a 21 cm map of the Milky Way using a 9-foot Satellite TV dish antenna. This work was discussed in the August 96 Sky and Telescope.

I hope that this information is helpful.

Paul Butterworth and Leonard Garcia
for the Ask a High-Energy Astronomer Team

QUESTION:
I teach astronomy, and a student once asked me which there are more of in our galaxy -- Pop. I stars or Pop. II stars. I wasn't sure! Do you know? More specifically, what is the ratio of the number of Pop. I stars to the number of Pop. II stars in the galaxy?

According to 'Galactic Dynamics' by Tremaine and Binney (1987: Princeton) the ratio of luminosities of the spheroid population to the disk population is 1/30. In order to translate to numbers of stars, we can use the fact that the mass/light ratio of the spheroid is greater than that for the disk (they suggest values of 5 and 12, respectively). Also, the mass distribution of disk stars has a large contribution from the more massive stars, which are not present in the halo. Taking all these things into account is tricky, and the best I can do is to suggest that the disk population probably wins, but not by a large factor (less than 10).

I hope this helps!

Tim Kallman
for the Ask a High-Energy Astronomer Team

QUESTION:
I heard on the news tonight that a dark matter galaxy recently collided with the milky way. And that astronomers aren't worried because it has previously happened ten times. I can't find any more information about this anywhere on the Web. My astronomy knowledge is somewhat limited. I am not a student, I am a screenwriter.

The announcement of a small galaxy colliding with ours was made by astronomer Rosemary Wyse at the annual meeting of the American Association for the Advancement of Science in Philadelphia in February.

The dwarf galaxy is not a "dark matter" galaxy, but rather the dark matter mixed in with the rest of the material in the dwarf galaxy appears to be holding it together under the gravitational tidal forces exerted by our galaxy. The proximity of this dwarf galaxy, however, will allow astronomers to better study the nature of dark matter.

Try http://www.seds.org/messier/more/SagdEg.html for a description of the Saggitarius Dwarf in more details and http://www.sciam.com/1998/1098issue/1098laham.html which is an article on hard-to-see galaxies behind the Milky Way.

Jim Lochner

Other Galaxies

QUESTION:
How many galaxies are out there ? And how do you know?

It is believed that there are a few billion galaxies in the Universe. This is based on observations of how many galaxies we see in a small part of the sky scaled to give us an estimation of how many galaxies the would be in the whole sky.

Hope this helps,

Laura Whitlock

QUESTION:
What different types of galaxies are there? What are their similarities and differences?

There are indeed different types of galaxies. The main types are spiral galaxies (like our own MilkyWay), elliptical galaxies and irregular galaxies. An irregular galaxy has an undefined shape and has lots of young stars, dust and gas. A spiral galaxy is shaped like a disk, usually with a bulge in the center and with arms that spiral outwards as the galaxy rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. Elliptical galaxies contain older stars and very little gas and dust. They can be different shapes ranging from round, to flattened, elongated spheres.

J. Allie Cliffe

QUESTION:
First, why do stars clump together into arms in spiral galaxies and are there commonly a specific number of prominent arms in a typical spiral galaxy? Is there a correlation between star masses, rotational speeds, galactic size, etcetera and the spiral arms?

Second, and this will settle an argument with a coworker :). What is the typical evolution of a cluster of stars forming into a galaxy? My friend seems to think that we start with spirals then move to disks. He uses as an analogy, chocolate syrup mixing in a glass of milk (where the chocolate particles are stars and the milk is space). I think this is wrong. Is he correct? And if he is, why does this happen? Or if he is wrong then what is the right answer?

The answers to your two questions are, not surprisingly, related. What causes spiral arms are density waves propagating through the stars and gas of the galaxy. This means that stars in an arm may not be in the arm after the density wave has moved on. What exactly causes these density waves is not known. Because of the increased density of gas in the arms, star formation in a spiral galaxy is concentrated in them, and so newer, bigger, and brighter stars tend to be in the arms.

As for the history of galaxies, the current thinking is that galaxies that have a relatively low total angular momentum form elliptical galaxies, and high angular momentum galaxies form spirals. Spirals probably start more like ellipticals, then collapse down to a disk (and a more spherical center bulge), and then the spiral arms form. Spirals all tend to be similar in mass, as opposed to the elliptical galaxies which vary in size from very small to the largest of galaxies. How these galaxies continue to develop is a topic for speculation, or you can ask me again in 5 or 10 billion years.

Eric Christian

QUESTION:
What exactly happens when two galaxies collide?

Collisions of galaxies are tremendous things (a galaxy is a LOT bigger than anything on Earth that you can imagine colliding!) and generate a lot of energy, heating and mixing up the gases in the two galaxies, making a good place for star formation. Unlike car collisions, galaxies collisions take a very long time - as many as a billion years or more for large galaxies!

There is lots of interesting information to be found on the web about what happens when galaxies collide, and even some recent images of galaxies that are in the process of colliding. A good brief explanation and images can be found at:

http://astrowww.phys.uvic.ca/~patton/openhouse/collisions.html

Or, you can take a look at the NASA press release from a Hubble Space Telescope image of two galaxies colliding at:

http://pao.gsfc.nasa.gov/gsfc/weekly/1997picks/97-138P.htm

Galaxy collisions are complex interactions and there are many people trying to figure out how galaxies interact when they get close enough together, and how they affect each other. One of the ways scientists do this is by studying numerical simulations of colliding galaxies. The simulations capture much of the important physics but can be run on a much faster timescale. An example of this research can be found at:

http://www.sdsc.edu/IOTW/week47/iotw.html

There are also movies of computer simulations at the University of Victoria site (the first one above).

Also: http://www-astronomy.mps.ohio-state.edu/~ryden/ast162_7/notes30.html

Allie Cliffe and Jim Lochner

QUESTION:
After reading a book about galaxies and learning that galaxies can sometimes change shape when one galaxy brushes past another galaxy, one of the students in our Third Grade asked if part of a galaxy can break off and join another galaxy when they brush past each other?

This is a very good question. Unfortunately I can't give a definitive answer. Here is what I can tell you: galaxies consist of 3 kinds of material: gas, stars, and 'dark matter' (material that we know must exist because its gravity is needed to hold the galaxy together, but we can't observe it directly). When 2 galaxies interact at a distance, they affect each other through their gravitational forces. These are of 2 types: first is the ordinary gravitational attraction which holds us onto the earth, and which holds solar system together; second is the tidal force, which is due to the fact that gravity decreases with distance. The tidal forces due to the moon and the sun are responsible for the earth's tides. If they were very much stronger, they could actually rip the oceans off of the earth, or rip the earth apart. There is no danger of this, but it can happen in galaxy interactions -- the tidal forces can disrupt one of the galaxies, or remove the gas from one of them. This has been suggested as a way of explaining why some galaxies (ellipticals) have little gas while others (spirals) have a lot more. In this case some of the gas is probably transferred to the bigger galaxy. Other possible interactions include total disruption of one of the galaxies, or merging of the two galaxies. Which of these occurs depends on how closely the galaxies approach each other and the masses of the two galaxies.

I hope this helps,

Tim Kallman
for the Ask a High-Energy Astronomer Team

QUESTION:
I would like to know about the history in brief about the discovery of galaxies in the Universe. If this seems to be a lengthly question please give me references.

Immanuel Kant, who was a famous philosopher, was the first to suggest around 1755 that the spiral nebulae were island universes. Prior to this century there was no distinction made between the things that today we call nebulae, such as the star forming region in Orion or the crab, which are luminous gas clouds within our galaxy, and other galaxies. This is exemplified in the Curtis Shapley debate of 1920. You can read about how this was resolved and other related topics at:

http://astro.gmu.edu/classes/a10695/notes/l17/l17.html

http://www.cnde.iastate.edu/staff/jtroeger/galaxies.html

I hope this helps,

Tim Kallman

QUESTION:
What is the error factor in the distances to galaxies? For instance, the Andromeda galaxy (M31) is 2.8 million light years. How accurate is that? Is the error plus or minus 1 million light years or what? And does that error factor get much worse the farther out we go?

Nearby galaxies, where you can see individual stars, probably have about a 10% distance uncertainty.

For galaxies at distances where you can't see individual stars, the distance is found by multiplying the redshift by a number called the Hubble Constant H0. The value of H0 is a contentious issue, but the two extreme camps are arguing for ~55 or ~70 km/s/Mpc, which means that there is about a 30% range in how far away people think any given galaxy is. The accuracy with which the redshift is measured doesn't depend much on the distance to the galaxy, so this is a constant factor: you know that one galaxy is 3.0 times as far away as another, and not 3.1 or 2.9 times.

At distances which are a good fraction of the age of the universe away, the question is how constant the Hubble constant is. Depending on how much mass there is in the universe (and thus how much the universe has been slowed down by gravity--so how much faster it was expanding in the early days) this can add an additional uncertainty range of 50%.

David Palmer

QUESTION:
Do you think you could send me any links to good web-pages that have information on Lenticular Galaxies or do you know where I can get any good information on them at?

I am not an expert on galaxy morphology, but here is my understanding.

Lenticular galaxies are best described as 'spiral galaxies without the spiral'. The spirals highlight the places where bright new stars are forming, but lenticular galaxies (like ellipticals) have lost the interstellar gas which forms new stars. Lenticular galaxies are most often found in dense clusters of galaxies, so the most likely conclusion is that collisions with other galaxies or intergalactic gas clouds have stripped the gas out of these galaxies.

Hubble classified the shapes of galaxies in a 'tuning fork' diagram, with spirals and barred spirals as the tines of the fork, and elliptical galaxies as the handle. Lenticular galaxies on this classification system are where the tines meet the handle. Go to http://www.seds.org/messier/lenticul.html to see some images of lenticular galaxies.

A search engine may turn up over references to lenticular galaxies. Since lenticular galaxies are not a topic popular on the 'fringe', most of these pages should be reasonably accurate. As always, consider the source of the information before deciding how much to trust its validity.

David Palmer
for Ask a High-Energy Astronomer Clusters of Galaxies

QUESTION:
I'm curious as to the recent need for teraflop and petaflop computers for the sole purpose of calculating the evolution of clusters (gravitational pulls, twin star formations, and other collisions).

When we finish calculating many of these virtual clusters of galaxies, will we be able to understand if our cluster is really the center of the Universe and if it is truly an average cluster? Has this already been explored?

Thank you for your question about evolution of galaxy clusters, and the need for fast (or special purpose) computers for this work.

The basic reason why the investigation of the dynamical evolution of galaxy clusters (as well as the evolution of single galaxies, or even globular clusters) is so computer intensive actually is due to a fundamental mathematical property of the equations that determine this evolution. The gravitational attraction between all objects is described by Newton's Laws, which you are probably familiar with. One law states that the gravitational force between two objects is a constant multiplied by the product of the two masses, divided by the distance separating the objects squared. In most of the solar system examples we are presented with on a 'day to day' level, the system can be described as two bodies. For each of the planets, we can treat their orbital evolution largely as if they were a single object in orbit about the Sun (the other planets produce only minor perturbations to this simple two-body orbit). Likewise, the Moon's orbit about the Earth can be treated largely as a two-body problem, since the distance between the Earth and Moon is much smaller than that between the Earth-Moon system and the Sun. Mathematically, the two-body problem is one that we refer to as 'integrable'. What this means is that it is possible to write down the solution to the equations of motion in closed form. Then for any set of initial conditions, we can use this closed form solution to determine the positions and velocities of the two bodies for all time.

When even one more body is added to the mix, the problem becomes 'non-integrable'. This has two important consequences. The first is the equations that determine the evolution are no longer in closed form. The second is that the system can now have parameter ranges for which the evolution is extremely sensitive to the initial conditions of the system. Very small changes in the initial conditions (positions and velocities) can lead to drastically different evolutions. Putting these two consequences together, you can probably see now why one needs a lot of computer power: galaxy clusters are comprised of numerous objects (galaxies) which are themselves made up of individual stars, interacting with each other. There are clever ways to make the calculation of the cluster evolution less computationally intensive, such as concentrating only on the interactions of nearest neighbor stars, and treating the contribution from the numerous more distant stars as a smooth gravitational potential. You still need to have a lot of computer power to do this. The sensitivity to initial conditions means that researchers often try a very large number of initial conditions so they can get an idea of the statistical behavior of the interactions.

To answer your specific questions: the need for teraflop or faster computers to do these calculations is not recent. However, the development of special purpose computers (that are hard wired to do nothing but the cluster evolution calculation) and novel ways of networking computers to achieve greater speeds, are currently very active areas of computational astrophysics research. The sophistication of the cluster evolution models is constantly growing. None of these calculations are aimed at trying to determine if our cluster is the center of the Universe. One of the fundamental assumptions of modern cosmology is that no single location in the Universe is special, and that there is no meaning to the concept 'the center of the Universe.' However, the average observer in any location in the Universe would observe galaxies to be receding from her position, and so might erroneously suppose herself to be at the center of the Universe. In any case, the dynamical evolution of the cluster is a local phenomenon, not connected to the overall expansion of the Universe.

Sorry if this is long-winded, but your questions touch on topics which are not easy to answer without going into the details.

Cheers,